The present invention relates to coatings for ionographic or electrophotographic, including digital and image on image, imaging and printing apparatuses and machines, and more particularly is directed to coatings for donor members and particularly donor members including electrodes closely spaced therein to form a toner powder cloud in the development zone to develop a latent image. The present invention is directed, in embodiments, to suitable conductive and semiconductive overcoatings, especially for donor member or transport members like scavengeless or hybrid scavengeless development systems.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two component and single component developer materials are commonly used for development. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface, the toner image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
One type of development system is a single component development system such as a scavengeless development system that uses a donor roll for transporting charged toner (single component developer) to the development zone. At least one, and preferably a plurality of electrode members, are closely spaced to the donor member in the development zone. An AC voltage is applied to the electrode members forming a toner cloud in the development zone. The electrostatic fields generated by the latent image attract toner from the toner cloud to develop the latent image.
Another type of development system is a two component development system such as a hybrid scavengeless development system which employs a magnetic brush developer member for transporting carrier having toner (two component developer) adhering triboelectrically thereto. A donor member is used in this configuration also to transport charged toner to the development zone. The donor member and magnetic member are electrically biased relative to one another. Toner is attracted to the donor member from the magnetic member. The electrically biased electrode members detach the toner from the donor member forming a toner powder cloud in the development zone, and the latent image attracts the toner particles thereto. In this way, the latent image recorded on the photoconductive member is developed with toner particles.
Coatings for donor members are known and may contain a dispersion of conductive particles in a dielectric binder. The desired volume resistivity is achieved by controlling the loading of the conductive material. However, very small changes in the loading of conductive materials at or near the percolation threshold can cause dramatic changes in resistivity. Furthermore, changes in the particle size and shape of such materials can cause wide variations in the resistivity at constant weight loading. A desired volume resistivity of the coating is from about 10.sup.7 to about 10.sup.13 ohms-cm, and preferably from about 10.sup.8 to about 10.sup.11 ohms-cm. If the resistivity is too low, electrical breakdown of the coating can occur when a voltage is applied to an electrode or material in contact with the coating. Also, resistive heating can cause the formation of holes in the coating. When the resistive heating is too high, charge accumulation on the surface of the overcoating can create a voltage which changes the electrostatic forces acting on the toner. The problem of the sensitivity of the resistivity to the loading of conductive materials in an insulative dielectric binder is avoided, or minimized with the coatings of the present invention.
Currently, ceramic materials are used for donor members such as donor members used in hybrid scavengeless development apparatuses. Several problems are associated with use of ceramic materials including non-uniform thickness, non-uniform run-out, pin hole defects, and rough surface finish. These problems can result in print defects. The problems are not easily overcome because they may be related to the deformation of substrate during high temperature thermal spray coating of ceramic materials. Grinding the ceramic coatings is needed to provide the desired surface finish. This additional, difficult, and low yield manufacturing process results in high unit manufacturing costs. In addition, the electrical conductivity of ceramic coating cannot be easily controlled and reproduced.
However, with the coatings of the present invention, the above problems with use of ceramic materials are reduced or eliminated.
Other coatings for donor members are described in the literature including the following patents.
U.S. Pat No. 5,300,339 discloses a coated toner transport roll containing a core with a coating thereover.
U.S. Pat No. 5,172,170 to Hays et al., discloses an apparatus in which a donor roll advances toner to an electrostatic latent image recorded on a photoconductive member. The donor roll includes a dielectric layer disposed about the circumferential surface of the roll between adjacent grooves.
U.S. Pat No. 5,386,277 discloses a coated toner donor member wherein the coating comprises oxidized polyether carbonate.
U.S. Pat No. 5,448,342 discloses a coated transport means comprising a core and a coating comprising charge transporting molecules and oxidizing agent or agents dispersed in a binder.
U.S. Pat No. 4,338,222 discloses an electrically conducting composition comprising an organic hole transporting compound and the reaction product of an oxidizing agent capable of accepting one electron from the hole transporting compound.
U.S. Pat No. 5,587,224 discloses a coated donor roll comprising a core with a coating comprising a photolysis reaction product of a charge transporting polymer and a photoacid compound.
U.S. Pat No. 5,264,312 discloses a process for preparing a photoreceptor by forming a coating following curing. The coating comprises an electroactive material dispersed in a polymerizable film forming monomer, which is first polymerized into a solid matrix.
U.S. Pat No. 5,731,078 discloses a coated donor roll comprising a substrate with a coating comprising a charge transport molecules, the metal salts of an organic acid and a polymer binder.
U.S. Pat. No. 5,853,906 discloses a conductive coating comprising an oxidized oligomer salt, a charge transport component, and a polymer binder, for example, a conductive coating comprising an oxidized tetratolyidiamine salt.
There exists a need for a donor member coating which provides conductivity or resistivity within a desired range, minimizes residue voltage, is relatively uniform and virtually free from defects and pinholes, provides good wear resistance for up to several million, for example 10 million copies, provides consistent performance with variable temperature and humidity, is low in cost and is environmentally acceptable.